The present invention relates to an oil pipeline which is exposed to cold operating conditions, particular to a subsea pipeline which is subject to cold water temperature and also to a subsea pipeline exposed to elevated subsea pressure.
Insulated oil pipeline is used in subsea transport of oil and in extraction of oil. In an oil field, when production initially starts and after production has halted for an interval, insulated subsea pipeline is at the cold water temperature. The reduced temperature of effluent in the pipe, such as residual oil and wax, will likely be reduced below the temperature of wax formation in the pipeline, which will lead to pipeline blockage, for example, between a subsea well and the receiving facility, such as a floating facility.
There are several known solutions to the problem of a chilled pipeline.
In subsea pipeline and riser systems, there is a requirement to keep the pipeline hot during normal operations and also during transient conditions, such as start up and cool down. This has traditionally been achieved by use of a passive insulation system around the pipe, typically using a low density material, such as polyurethane foam or a substance known under the trademark Rockwool. Passive insulation systems are limited in two main respects. The amount of insulation that can be applied to a pipeline has a practical limit because the effect of the applied material in reducing heat loss has a log normal distribution, providing diminishing return from simply adding more material. Secondly, passive insulation of a pipeline cannot assist during start up or restarting when the pipe is cool. The insulation can only keep the pipeline warm and it cannot input heat to a cold pipeline. For example, it is known that most pipeline blockages due to the presence of hydrates occur after an extended shut down.
One usual solution to the blockages is to introduce chemicals in the pipeline during start up. To avoid wax formation during start up, operators inject methanol or glycol in the effluent to reduce its viscosity.
Another possible solution is to heat the pipe before start up using an active heating system. This is particularly useful either for very long tie backs or for onerous thermodynamic conditions, particular experienced in riser systems. One type of active heating system uses electrical heating of the pipe, with obvious attendant expense. In another, more typically used system, the pipe has an annulus around it, and heating is achieved by transporting a heated fluid along the pipe annulus. More preferable in some current systems is the use of a small diameter tube(s) passing through the annulus around the oil carrying pipe, which is typically wound around the main pipe. The small tube(s) carries hot water which is pumped through it.
Both the electrical power heating system and the heated fluid transport down the annulus are technically feasible, but require significant amounts of energy to be effective. Further, they are thermally inefficient since they try to heat the pipe from the outside in. Such inefficiencies limit the use of these systems, both commercially and technically.
Current active heating systems that use small hot water carrying tubes require that the tubes be designed to resist being crushed. Hoses made of carbon steel or super duplex are used. When the tubes are comprised of thermoplastic material, reinforcing elements are added to protect these hot water tubes. The reinforcing elements either resist the crushing or are able to transmit the crushing force to the main pipeline. For example, U.S. Pat. No. 6,102,077 and references therein cited discloses a flexible pipe with thermoplastic hot water hose wound around the main pipe. Filler is added between each winding of the hot water hose to transmit the crushing force of the subsea environment to the main flexible pipe. Where rigid subsea pipes are used, the thermoplastic material hot water hoses are protected by an outer steel pipe called the carrier pipe. Alternatively, the hot water can be injected through the annulus between the inner pipe and the carrier pipe, avoiding need for use of a hot water tube.
The above described conventional techniques for heating the pipe have the drawback that there are significant heat losses toward the exterior of the pipe, and the energy level required for heating the pipe is high. For example, for a start up with a system wherein hot water is passed through the annulus between the outer and inner pipes, it may take up to ten days to heat the pipe sufficiently for efficient transmission of oil.